Recently, with increasing awareness of environmental protection, the demand on renewable energy is growing. Among various renewable energy sources, solar energy is expected to replace fossil fuel as a new energy source because it provides clean energy without depletion. The solar energy may be converted into electric energy without generating contaminants. In other words, the solar energy source is the most viable renewable energy source.
Generally, a solar cell is used to convert solar energy into electric energy. The conventional solar cell is made of semiconducting materials. In particular, silicon-based solar cell is the mainstream in the market. The photoelectric conversion efficiency and the cost-effectiveness of the solar cell are gradually improved. However, the photoelectric conversion efficiency and the cost-effectiveness of the conventional solar cell are still unsatisfied so far. Consequently, many academic institutions and manufacturers devote much effort in improving the original solar cell configurations and looking for novel solar cell configurations in order to increase the performance and reduce the fabricating cost. For example, a dye-sensitized solar cell (DSSC) is one of the candidate solar cells because the fabricating cost of the dye-sensitized solar cell is as low as one tens to one fifth of the fabricating cost of the conventional silicon-based solar cell. Moreover, since it is not necessary to produce the dye-sensitized solar cell under high-temperature vacuum environment, the production conditions of the dye-sensitized solar cell are less stringent than the conventional silicon-based solar cell.
The dye-sensitized solar cell is a photoelectrochemical system that uses the photoexcitation of a dye-based photosensitizer to generate photocurrent resulting from a chemical reaction. Generally, the dye-sensitized solar cell includes a substrate, a transparent conductive film, a semiconductor film, a dye, an electrolyte and a counter electrode. The working principles will be illustrated as follows. Firstly, a titanium dioxide or zinc oxide semiconductor film is coated with a special light-absorbing dye. When the dye is irradiated by sunlight, electrons of the dye are excited from a ground state to an excited state. The excited electrons may be injected into the conduction band of the semiconductor film so as to become free electrons. These free electrons may flow out through conductive glass or conductive plastic (conductive polymer), which is connected with the semiconductor film. Consequently, a current is generated. The electrolyte is located at the other side of the dye. The dye molecules that lose the electrons may receive electrons from the electrolyte so as to be restored to its original state. Then, the electrons flowing from the dye pass through a load and reach a platinum electrode, which contacts with the electrolyte. Consequently, the electrolyte receives these free electrons, and a complete loop is created. In the dye-sensitized solar cell, the coating manner of the photosensitive dye and the structure of the photosensitive dye may influence the photoelectric conversion efficiency and the stability of the dye-sensitized solar cell. Moreover, the selection of the photosensitive dye is highly related to the cost of the dye-sensitized solar cell and even related to the development potential of the solar cell.
Organic photosensitive dyes have been emphasized in development of solar cells for their high absorption coefficient, facile modification and adjustable photophysical properties. The inventor has described a photosensitive porphyrin-based dye in US 2010/0125136 A1. Porphyrin is viewed as artificial chlorophyll. Chlorophyll is a green pigment found in plants and allows plants to absorb light to initiate photosynthesis, i.e. producing oxygen and carbohydrates from carbon dioxide and water. The porphyrin plays a similar role in the dye-sensitized solar cells, e.g. converting energy of visible light and near-infrared light into electric energy. The advantages of using the porphyrin molecules in the dye-sensitized solar cells include appropriate energy level of the excited electrons to the utilization of titanium dioxide, wide absorption range of sunlight, and longer lifetime of the excited electrons. However, the conventional solar cells using the porphyrin do not gain satisfied benefit. It is found that the porphyrin molecules are easily aggregated to affect the photoelectric conversion efficiency.
Therefore, novel photosensitive porphyrin-based dyes are developed in the present disclosure to improve properties of the photosensitive dyes for a photoelectric converting device such as a dye-sensitized solar cell. The balance between the photoelectric conversion efficiency, the stability of the solar cell, the ease of synthesis and the cost effectiveness may increase the commercial competition of the dye-sensitized solar cell.